Oil wells which produce fluids containing an unacceptably high amount of water are often treated by introducing a gel into highly permeable formations to block the flow of the water. This normally results in reducing water production and increasing drawdown of the fluid column. In many cases, however, oil production does not increase, and in fact may even decrease, as a result of the oil now having to flow through the matrix rather than through the more permeable formation of its former path. It would therefore be beneficial to be able to shut off the permeable zone in order to reduce water production and to subsequently open it again at a reduced permeability level so as to increase oil production.
Until now such operations have not been commercially possible because the gels used to close off a formation completely block the fractures in the formation, making the formation impermeable. If it is desired to open the formation after a period of time, known methods of treatment can be carried out to break the gel while in place or the gel can be selected so as to eventually degrade due to the effects of time and temperature. In either case the original permeability of the formation is restored, not a different permeability.
Many different gel systems have been employed in the treatment of subterranean formations, some utilizing various polymeric materials and crosslinking agents. It is also known to include a breaker in the gel composition so that the breaker can degrade the gel after a predetermined period of time instead of having to treat the gel in place. Breakers have included many different materials, including various oxidizing agents, strong chemicals, chelating agents, organic chemicals and chemically associated compositions, and in many cases two or more ingredients are used as breakers in a gel system.
There is, however, no gel-breaker system which functions other than to completely break down the gel after a period of time, making it very difficult to treat highly permeable formations in any different manner than the manner in which it has been treated in the past.
The use of ethylenediaminetetraacetic acid (EDTA) in a gel system is not unknown. For example, U.S. Pat. No. 4,428,432 discloses the use of EDTA as an additive in a crosslinked acid polymer gel system in order to reduce the tendency for solids to form upon the breaking of the crosslinked acid polymer. The crosslinked acid polymer breaks down as the result of time and temperature, not as a result of the EDTA.
In U.S. Pat. No. 4,644,073 the use of EDTA as a sequestrant is disclosed. It is introduced into the formation being treated as a spacer solution between cycles of introduction of aqueous polymer and chromium propionate. U.S. Pat. No. 4,636,572 teaches a similar use.
In U.S. Pat. No. 4,488,601 EDTA is also disclosed as providing the function of a sequestering anion in a spacer between the cycles of introduction of polymer and crosslinking agent. In addition, it is disclosed as being used as a retarding anion in the preparation of crosslinking agents.
The use of EDTA as a single component breaker in a gel system of the type described, however, is not suggested in the prior art.